It seems that every week, we hear about a new exoplanet that's located smack-dab within its solar system's habitable zone — that warm and cozy Goldilocks area that's suitable for the emergence of life. In turn, some skeptics have contended that the parameters defining the habitable zone are far too liberal and too often misunderstood, or that they're based on criteria that were developed 20 years ago — a time before the first exoplanet was even discovered.

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In their zeal to correct this, the original team that worked to develop the boundary recently rejigged the parameters — and they've placed the Earth right on the inner edge of being considered "habitable".

This news comes to us from Space.com's Clara Moskowitz, who describes the work of research team leader, Ravi Kumar Kopparapu of Penn State University. The revised definition, which is set to appear in an upcoming edition of the Astrophysical Journal, will have a significant impact on the number of exoplanets that are categorized as being within a habitable zone.

For a planet to qualify as being habitable, it needs to meet certain conditions, including the ability to support liquid water on the surface, and atmospheric pressure high enough for water to exist without boiling off to vapor. It also needs an atmosphere that can alter the transfer of radiation to and from the surface (à la greenhouse effect). For a planet to have these characteristics, it obviously needs to be in the right place.

But as Kopparapu's work now reveals, the habitable zone wasn't exactly where astrobiologists and astronomers thought it was — but they weren't off by much. Moskowitz explains:

The new definition of the habitable zone is based on updated atmospheric databases called HITRAN (high-resolution transmission molecular absorption) and HITEMP (high-temperature spectroscopic absorption parameters), which give the absorption parameters of water and carbon dioxide - two properties that strongly influence the atmospheres of exoplanets, determining whether those planets could host liquid water...

...The new definition isn't radically different from the old one. For example, in our own solar system, the boundaries of the habitable zone have shifted from between 0.95 astronomical units (AU, or the distance between Earth and the sun) and 1.67 AU, to the new range of 0.99 AU to 1.7 AU.

Consequently, the Earth appears to sit quite close to the inner edge of the habitable zone — an unexpected revelation!

As a final note, it's important to remember that the "habitable zone" definition still does not take into account feedback effects from clouds and other local phenomenon, which will all affect a planet's habitability.